The structure that controls which substances enter or exit a cell is the cell membrane, also known as the plasma membrane. The cell membrane is primarily composed of a phospholipid bilayer with embedded proteins. This structure provides a semipermeable barrier, allowing certain molecules to pass while restricting others.
Key features of the cell membrane include:
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Phospholipid Bilayer: The basic framework of the membrane, with hydrophobic (water-repelling) tails facing inward and hydrophilic (water-attracting) heads facing outward.
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Proteins: Various proteins are embedded in the membrane, functioning as channels, transporters, receptors, and enzymes. These proteins help facilitate specific substances' movement across the membrane.
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Selective Permeability: The cell membrane is selective, allowing some substances (like small, nonpolar molecules) to pass through easily while blocking others (like ions or large polar molecules) without the aid of transport mechanisms.
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Transport Mechanisms: Specific processes such as diffusion, osmosis, facilitated diffusion, active transport, and endocytosis/exocytosis help regulate the movement of materials in and out of the cell.
Overall, the cell membrane plays a crucial role in maintaining the internal environment of the cell and enabling communication and interaction with the external environment.